In precision circuits where the values of resistors, capacitors, transistors, and other components whose values in actual circuit implementations may be different from design specifications, either statically or transiently due to environmental conditions such as temperature, calibration of such values is of paramount importance. When a component value is calibrated and an error is detected, a physical component value can be changed or “trimmed” accordingly to compensate for an error, or the circuit components can be left alone and the error corrected or compensated for either by built-in compensation circuitry or computation logic or by external compensation. In cases of unacceptable errors or a lack of suitable compensation mechanism, the circuit may need to be discarded.
Calibration may be performed in a variety of ways. Two general types can be defined: background calibration; and foreground calibration. In foreground calibration, operation of the circuit is stopped while calibration is carried out, while in background calibration, circuit operation continues while calibration is carried out. Foreground calibration has the inherent advantage that test signals can be injected into the circuit without concern for the normal operation, and the desired values to be calibrated can be measured as a function of test signals. Measurement of the output values of the circuit as a whole or from selected circuit components or subcircuits is typically performed by additional testing logic, whether built-in or off-circuit. In the case of an ADC for example, a test analog signal can be swept though the dynamic range of the ADC, and the digital output response can be compared. Additionally or alternatively, key components can be fed specific test signals, and responses measured. Such test signals and response measurement are normally done by dedicated circuitry not involved in the normal operation of the circuit. In the case of calibrating a device, such as an analog-to-digital converter (ADC), there is significant advantage to being able to maintain precision of the device through background calibration without removing the device from normal continuous use. However, it is inherently more challenging to measure circuit values without disrupting normal operation by the use of known rest signals.
In some types of precision circuit devices, such as pipelined ADC's, the devices comprise interconnected stages, such as first stage precision circuitry whose output is measured by second stage precision circuitry. In the case of pipelined ADC's, the first stage converts one or more most-significant bits (MSB) of the analog signal, generates from those bits an analog signal to be subtracted from the input analog signal, and the difference is amplified and measured in the next stage. The next stage measures the next lesser significant bits, and the number of stages may be two, three or more. The last stage is often essentially a flash ADC. Flash ADC devices gain complexity and consume power almost geometrically for each additional bit of resolution added. Pipelined ADC's allow greater resolution without geometrically increasing complexity and power consumption, with the trade-off that each stage beyond the first introduces a sample delay in the output of the ADC.